<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button>1</button>
    <button>2</button>
    <button>3</button>
    <button>4</button>

    <script>
        var btns = document.querySelectorAll("button");
        for (var i = 0; i < btns.length; i++) {
            btns[i].onclick = function () {
                alert(i)
            }
        }


        //    改造方法一


        var btns = document.querySelectorAll("button");
        for (let i = 0; i < btns.length; i++) {
            btns[i].onclick = function () {
                alert(i)
            }
        }


        
        //    改造方法二   闭包的思想

        // var btns = document.querySelectorAll("button");
        // for (var i = 0; i < btns.length; i++) {
        //     btns[i].onclick = (function (i) {
        //         alert(i)
        //     })(i)
        // }








    </script>
</body>

</html>